Colorado......
I see on the **************** forum (John Deere) they list your 4100 as a medium frame CUT (MCUT). Along with the JD755 (20hp)......
My JD855 (25hp) is listed as a large frame CUT (LCUT) I know there are a lot of compatable parts between the 755 - 855 tractors. They are very close to the same size.
I bought a well used Speeco PHD, I presume a full size unit, cleaned it up and it runs fine. However...... In my rocky ground I learned it is a good idea to make yourself a handle that you can bolt to the auger bit, in place of the PHD to un-screw the bit out of the ground........... It isn't hard to make one, it isn't hard to remove the bit from the ground, but of course a pain in the neck to take the time to mess with..........
Good luck.